You can spend plenty of time getting your makeup just right, only to notice that it looks different a few hours later. Foundation may start disappearing around the nose, blush can become less noticeable, and areas that looked smooth in the morning may look a little uneven by the afternoon.
A good setting spray can help, but choosing one isn't as simple as picking the product with the biggest “long wear” claim. Your skin type, preferred makeup finish, weather, and the type of makeup you normally wear all matter.